a long periodic table showing, from left to right: the s-, d-, f-, and p-blocks. the f-block, normally shown as a footnote, here splits the d-block into two. this splitting is not universally agreed on; although it is the more common form in the literature, there is substantial opposition to it from many sources focusing on the placement of elements in the periodic table.
a block of the periodic table is a set of elements unified by the orbitals their valence electrons or vacancies lie in. the term appears to have been first used by charles janet. each block is named after its characteristic orbital: s-block, p-block, d-block, and f-block.
the block names (s, p, d, and f) are derived from the spectroscopic notation for the value of an electron's azimuthal quantum number: sharp (0), principal (1), diffuse (2), or fundamental (3). succeeding notations proceed in alphabetical order, as g, h, etc.